The present invention relates to a controlling device for conversion of DC power to sine wave AC power, which is applied to control and protect DC/AC converter.
It is known that the general DC/AC converter is a simple uninterruptable system. A battery or other DC power supply provides power for the converter. According to the alternating A phase and B phase generated by PWM (wave width modulation ) circuit, the power transistor is driven and serially connected to a middle output transformer to output AC power for external use. The output AC wave form is square wave. The PWM circuit and A, B phase drive circuit may have the following abnormal state during driving of the power transistor:
A phase and B phase are both positive, so that the magnetic fields of the A phase and B phase conflicts with each other and the current abruptly increases to burn down the power transistor. A phase is normal, while B phase fails to work or A phase fails to work, while B phase is normal. This leads to serious magnetic deflection and great increment of driving current. This may burn down the power transistor.
One of A phase and B phase has too long wave width or is positive during a long period or is positive at normal time. This leads to electromagnetization of the silicon-steel plate, total loss of coil inductance, abrupt increment of current and burning down of power transistor.
The square wave generated by A phase or B phase will also generate a considerably strong electromagnetic wave which will interfere with other peripheral electric appliance. For example, the television will suffer interference of oblique stripes.